Wet strength is an important characteristic of paper and tissue products, and in non-woven products. Wet strength of such products can be increased by using wet strength additives. The most widely used wet strength additives for the paper industry are melamine-formaldehyde resins, urea-formaldehyde resins and poly(amino)amide epichlorohydrin, PAE (see Espy, TAPPI Journal, 78, 90 (1995)). There is a tendency, however, to move away from such oil-based chemicals, because they are not renewable and have a poor biodegradability. The use of PAE is under pressure for several reasons: high price of PAE itself, poor repulpability and biodegradability of the paper product, and presence of toxic monomers and residues in the wet strength resin. The use of PAE could be avoided by providing cationic charge in the fibre itself and adding carboxylated or oxidized carbohydrates to these fibres. These carbohydrates have the desired renewability and degradability and are capable of providing wet strength to paper products; see WO 01/83887.
Cationic cellulosic fibre is a known product. U.S. Pat. No. 4,505,775 discloses a cationic cellulose obtained by reaction of cellulose fibre with a condensate of epichlorohydrin and dimethylamine. The cationic fibre has improved dye retention characteristics. A more recent survey of cationic cellulose fibres by Gruber et al. in Cellulose Derivatives, Modification, Characterisation and Nanostructures, Ed. T. J. Heinze and W. G. Glasser, A.C.S., Washington D.C., 1998, pp. 94-106, shows that the current cationisation of fibres still uses epichlorohydrin-type reactions. Therefore, the currently known cationic fibres do not provide a satisfactory solution to the problem of providing paper and non-woven products with sufficient wet strength based on renewable and biodegradable materials.